Robust continuous-flow oxidation of aryl-alkyl sulfides to sulfoxides and sulfones over titanium silicalite-1 using diluted hydrogen peroxide

A continuous-flow oxidation of sulfides was developed using a titanium silicalite-1 (TS-1) catalyst packed in a silica gel column and a single-phase MeCN solution containing diluted aqueous hydrogen peroxide (H₂O₂ aq.). Methyl phenyl sulfide afforded methyl phenyl sulfoxide in 91% yield at a residence time of 6.5 min (flow rate: 0.20 mL min^-1), whereas increasing the H₂O₂ loading and reaction temperature afforded methyl phenyl sulfone in 99% yield at the same residence time and flow rate. The method was extended to a range of thioanisoles to afford the corresponding sulfoxides and sulfones, important motifs in pesticides and pharmaceutical intermediates. Notably, high yields were achieved with only 5-10% H₂O₂ aq., avoiding the use of more concentrated H₂O₂ aq. (>10%), which can accelerate oxidation but presents explosion hazards. The practicality of the system was further demonstrated by the long-term continuous oxidation of methyl phenyl sulfide to methyl phenyl sulfone, which maintained a yield of over 90% at a residence time of 13 min and a flow rate of 0.10 mL min^-1 using a 1.3 mL column reactor for 170 h, producing 15 g of product.